Read the text and answer the following questions. 1. What is the form of a ship’s propeller?

1. What is the form of a ship’s propeller? 2. Where is the propeller situated? 3. When were the first experiments with propeller carried out? 4. What was the speed of an Australian ship at Trieste in the 1820s? 5. What is the speed of modern ships? 6. When was the English ship Archimedes tested? 7. What accident helped the Archimedes to increase the speed? 8. What discovery was made by Smith?

7.24. Read the text again and tell: 1) about the operation of the ship’s propeller, 2) the idea of Bernouilli, a Swiss scientist, 3) the Austrian ship, 4) the Archimedes tested in 1838, 5) Smith’s discovery.

A Ship’s Propeller

All big ships are now driven by engines of some kind, and usually by steam engines or oil engines. These turn the propeller, which is in the water at the back of the ship. It is in the form of a screw (винт). When it turns, it pushes the ship forwards just as s wood-screw goes forwards into a piece of wood when it is turned. Our present propellers are the result of experiments made about 1835; but the idea was not new then.

Bernouilli was a Swiss scientist who described a screw for ships in 1752, but he could not make any real tests. At that time there were no good engines for ships, and his idea remained only an idea.

Seventy years later the same idea was again in the minds of engineers. An Austrian, Ressel, built a small ship at Trieste. It had a long screw at the back which drove the ship forwards at about 7 ½ knots. One knot is speed of 6,080 feet (1,852 metres) per hour. Some modern ships travel at 30 knots, so 7 ½ knots was not very fast. But the screw did its work, and Ressel was pleased. His experiment did not fail completely; but unhappily there was an explosion in the ship’s engines, and after that Ressel did not try again.

In 1836 an Englishman, Francis Pettit Smith, had a ship, the Archimedes, which had a metal screw at the back. It was too long in fact; but at that time engineers knew little about ship’s propellers.

The Archimedes was tested in 1838, and everybody was pleased when it moved forwards. It had reached a speed of 4 knots when suddenly a loud noise was heard. Smith stopped the engine and went to look at the screw. A bottle had got under it and had broken half of it off; the other half remained, but Smith was not sure that he could reach land again.

He could not just stay there, and so he started the engine again to see what would happen. The ship began to move slowly. The broken screw was not useless. Smith was pleased; but soon the ship was going so fast that he was very surprised.

Smith had made a discovery: a short screw worked well. Later he discovered that a short screw worked better than a long one. With a short one the Archimedes reached a speed of 13 knots, and it once sailed from Portsmouth to Oporto (Portugal) in 70 hours.

Today most big ships are driven by propellers. These are turned by oil engines in modern ships.